The present invention relates to a transparent BN-type ceramic material comprising the elements B, N, and Si, and to a method of producing the same.
Ceramic materials with superior transparency are expected to show rapid development in the future. Materials related to optical device technology, with light as the medium, are eagerly awaited. In addition, materials with the ability to transmit light and provided with the properties of ceramic, such as inherent heat resistance, corrosion resistance, heat stability and high hardness, are expected to find use in special applications in optical science, such as in various types of window materials and lenses for high temperature use. In addition, they are expected to be used as structural members for various types of equipment and devices for use in outer space.
The present invention relates to a BN-type ceramic material which is transparent and has superior heat resistance and can be used for various types of window materials for high temperature use, lenses for high temperature use, and as mask support material for X-ray lithography.
The following researches have been reported relating to ceramics containing boron (B), nitrogen (N), and silicon (Si).
K. S. Mazdiyasni and Robert Ruh &lt;J. Am. Ceram. Soc., 64 (7) 414-19 (1981)&gt; have reported an Si.sub.3 N.sub.4 -BN composite material prepared by the addition of 5 to 50% BN powder to Si.sub.3 N.sub.4 powder+6% CeO.sub.2 with hot-pressing. According to their report, this composite had a strength which was inferior to that of pure Si.sub.3 N.sub.4, but had improved electrical properties and thermal stress resistance.
T. Hirai, T. Goto and T. Sakai &lt;pp 347 to 358 in Emergent Process Methods for High Technology Ceramics, edited by R. F. Davis, H. Palmour and R. L. Porter, Plenum Press, New York, 1982&gt; have reported a synthesis of an amorphous Si.sub.3 N.sub.4 -BN composite by chemical vapor deposition. This material was synthesized from chemical vapor deposition (CVD) of SiCl.sub.4 +NH.sub.3 +H.sub.2 +B.sub.2 H.sub.6 at a total gas pressure of 30.about.70 Torr and at a deposition temperature of 1100.degree.-1300.degree. C. They reported obtaining a brown transparent deposit. They filed a Japanese patent application (Japanese patent application Laid-Open No. 58-145665) relating to the above research.
The research which is closest to the present invention is that which was reported by Hirai et al. and is fourd in the Japanese patent application.
In Japanese patent application Laid-Open No. 58-145665, a transparent Si.sub.3 N.sub.4 -BN-type amorphous material is proposed, which has high transparency, high heat stability, high thermal shock resistance and high chemical resistance. This material is an amorphous composite material comprising 30 to 70 wt. % of Si.sub.3 N.sub.4 and 70 to 30 wt. % of BN, which is obtained by simultaneous deposition using a chemical vapor deposition method.
This transparent material is transparent and has a brownish color. It is obviously extremely unsatisfactory as a transparent ceramic material. This fact is stated in the above-mentioned laid-open patent application (page 4 ) to the effect that "As the color changes from white to yellow and to brown, the transparency improves. In the case, especially, where it exhibits a brown color externally, superior transparency is obtained." This explanation clarifies the fact even further. Specifically, the material proposed in the above mentioned patent application is not a colorless transparent material, but is a brown, transparent material. Here, "being transparent" and "being brown" are inseparably connected characteristics.
For producing the above material, a method of producing a Si.sub.3 N.sub.4 -BN type amorphous material is disclosed, which is characterized in that a silicon element depositing source gas, a boron element depositing source gas, and hydrogen gas together with a nitrogen element depositing source gas, are introduced separately into a furnace maintained under reduced pressure, and a chemical vapor reaction is carried out at the synthesizing temperatures of these reactive gases, which is in the range of 1100.degree. .about.1300.degree. C., so that amorphous Si.sub.3 N.sub.4 and BN are simultaneously deposited on a substrate installed in the furnace.
In the future, in the industrial fields of innovative advanced technologies, particularly in the fast developing field of optical technology, an extremely important requisite for transparent ceramic materials, which are expected to find a great many applications, is colorless transparency. Therefore, the inventors of the present invention have conducted many different types of research for the development of colorless transparent ceramics.